BIOL 118 1st Edition Lecture 17 Outline of Last Lecture I In Bacteria Sigma Subunits Initiate Transcription II Most Bacteria Have Several Types of Sigma Proteins III What Occurs Inside the Holoenzyme IV Elongation Termination V Eukaryotic Promoters VI Transcription Initiation in Eukaryotes VII RNA Processing in Eukaryotes VIII The Discovery of Eukaryotic Genes in Pieces a Exons b Introns IX RNA Splicing X Adding Caps Tails to RNA Transcripts XI An Introduction to Translation XII Transcription Translation in Bacteria XIII Transcription Translation in Eukaryotes XIV How Does an mRNA Triplet Specify an Amino Acid XV What Happens to the Amino Acids Attached to tRNA XVI The Characteristics of Transfer RNA XVII What DotRNAs Look Like XVIII How Many tRNAs Are There XIX The Structure Function of Ribosomes XX Ribosomes and the Mechanism of Translation XXI The Phases of Translation a Phase 1 Translation Initiation i Translation Initiation in Bacteria b Phase 2 Translation Elongation i Is the Ribosome an Enzyme or a Ribozyme ii Moving Down the mRNA c Phase 3 Translation Termination i Release Factors Post translational Modifications Outline of Current Lecture I Introduction II Gene Regulation Information Flow These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII Mechanisms of Regulation An Overview Transcriptional Control of Gene Expression Translation Control of Gene Expression Post Translational Control of Gene Expression Control of Gene Expression in Bacteria Metabolizing Lactose A Model System Identifying Genes Under Regulatory Control Finding Mutants for a Particular Trait Replica Plating to Find Mutant Genes Different Classes of Lactose Metabolism Mutants Several Genes Are Involved in Metabolizing Lactose Transcriptional Regulation Mechanisms of Negative Control The Repressor The lac Operon How Does Glucose Regulate the lac Operon Why Has the lac Operon Model Been So Important Current Lecture Introduction A cell does not express all of its genes all the time Cells are very selective o About the genes they express o How strongly they are expressed o When they are expressed Gene Expression occurs when o A gene product is actively being synthesized o Used in a cell Regulation of gene expression is critical to o The efficient use of resources o Survival Gene Regulation and Information Flow Escherichia coli has served as o An excellent model organism o For the study of prokaryotic gene regulation Like most bacteria E coli can use a wide array of carbohydrates to supply carbon and energy Control of gene expression allows E coli to o Respond to its environment o Switch its use of sugars Gene expression in bacteria was predicted to be triggered by specific signals from the environment Mechanisms of Regulation An Overview Information flow occurs in three steps o DNA mRNA protein activated protein Genes can be under o Transcriptional control o Translation control o Post translational control All three types occur in bacteria Level of expression of different genes can be highly variable Variation in gene expression allows cells to respond to changes in their environment Transcriptional Control of Gene Expression Occurs when the cell does not produce mRNA for specific enzymes The cell avoids the production of these enzymes o By utilizing regulatory proteins that prevent RNA polymerase from binding to a promoter Is slow but efficient Translational Control of Gene Expression Allows the cell to prevent translation of an mRNA molecule that has already been transcribed Control can occur through many mechanisms o Regulatory molecules can speed up mRNA degradation o Translation initiation can be altered o Translation proteins can be affected Allows a cell to quickly change which proteins are produced Post translation Control of Gene Expression Occurs when the cell fails to activate a manufactured protein by chemical modification Provides most rapid response is energetically expensive Metabolizing Lactose A Model System E coli s preferred carbon source is glucose o It uses lactose only when glucose is depleted Before it can utilize lactose o E coli must transport it into the cell o The enzyme b galactosidase can cleave it to produce glucose galactose E coli produces o High levels of b galactosidase o Only when lactose is present in the environment Lactose acts as an inducer o A molecule that stimulates the expression of a specific gene Identifying Genes Under Regulatory Control Jacques Monod and Francois Jacob isolated and analyzed e coli mutants that could not metabolize lactose To find the genes that code for o B galactosidase o The membrane transport protein that brings lactose into the cell Finding Mutants for a Particular Trait Isolating mutants with respect to a particular trait is a two step process o Generate a large number of individuals with mutations at random locations in their genomes o Use genetic screening on the mutants to find individuals with defects in the process or pathway in question Replica Plating to Find Mutant Genes Replica plating is a four step process used to identify mutant cells o Grow bacterial colonies on master plate containing a medium with many sugars o Transfer cells from each colony to a piece of sterilized velvet o Transfer cells to a replica plate with a medium containing only lactose to screen for colonies that could not grow on lactose o Compare the colonies on the master replica plates Indicator plates allow researchers to directly observe mutants with metabolic deficiencies o Made it possible for them to identify colonies without functioning bgalactosidase Different Classes of Lactose Metabolism Mutants The three genes involved in lactose metabolism are lacZ lacY lacl Three classes of E col mutants defective in lactose metabolism were identified o LacZ mutants lack functional b galactosidase o LacY mutants lack the membrane protein galactosidepermease Cannot transport lactose into the cell o Lacl mutants are called constitutive mutants because when lactose is absent they produce B galactosidase Galactosidepermease Several Genes Are Involved in Metabolizing Lactose The lacZ lacY genes code for proteins involved in lactose metabolism o In the presence of lactose transcription of lacZ lacY is induced The lacl gene product serves a regulatory function o In the absence of lactose the lacl gene product shuts
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